Bird Technologies 21-87A-11-xx-T Series-Manual - Manual (Page 22)

TX RX Systems Inc. Manual 7-9100-5 10/17/11 Page 16
cavity selectivity which may be necessar y to
accommodate more closely spaced channels.
Changing the loss is accomplished by rotating the
coupling loops to change the coefficient of cou-
pling. Both loops are normally adjusted for a given
insertion loss setting. Most T-Pass cavities have a
Calibration Index label beside both loops that gives
a relative indication of their settings (see Figure
11). In actual practice, these marks are not accu-
rate enough for setting loss values consistently.
Two procedures are offered for setting the cavity
loss. Both procedures take advantage of the fact
that when a tee connector is placed on a single
bandpass or T-Pass loop, a rejection notch can be
observed across the tee. The depth of the rejection
notch is directly related to the loop's coefficient of
coupling.
The first procedure uses precision rotary attenua-
tors, a signal generator and an RF millivolt meter to
provide very accurate results. The actual loss set-
ting obtained when this procedure is carefully fol-
lowed will be within one tenth of a dB of the desired
value and the return loss will be 20 dB (1.25:1) or
better.
The second procedure uses a spectrum analyzer
and frequency generator and produces slightly less
accurate results. When this procedure is carefully
followed, the loss settings will be within two tenths
of a dB of the desired value and the return loss will
usually be -15 dB (1.5:1 VSWR) or better. The
advantage of this procedure is that it is much faster
to do, does not require precision attenuators and
will yield acceptable results in most cases.
Table 4 shows a reference chart for setting T-Pass
cavity loss with either procedure. The chart shows
the desired cavity loss settings and the reference
setting for both the T-Pass and bandpass loop
assembly. The reference notch depth for a given
loss is that which can be observed across a tee
connector connected to either loop assembly. Note
that the reference notch depths are slightly differ-
ent when the T-Pass loop assembly has a built-in
Calibration
Mark
Bandpass
Loop
T-Pass
Loop
Loop Locking
Screws (6 places)
Figure 11: Top view of T-Pass cavity.
Cavity Loss (dB)
Built-in Short
Circuit
Coupling Loop Type
TX RX Systems
Part Number
Reference Notch
Depth (dB)
1.25 No
T-Pass
Bandpass
3-3721
3-2294 -9
-17
1.25 Yes
T-Pass
Bandpass
3-2292
3-2294 -9.6
-16.7
1.80 No
T-Pass
Bandpass
3-3721
3-2294 -6.6
-14.6
1.80 Yes
T-Pass
Bandpass
3-2292
3-2294 -7.2
-13.9
Table 4: Cavity insertion loss reference loop settings.